This invention relates to a method of manufacturing an improved noise attenuation structure, more particularly to a method of manufacturing a structure having a first impervious facing sheet adhered to one surface of a first honeycomb core, a first perforate facing sheet with a first layer of porous fibrous material adhered to the other surface of the first honeycomb core, a second perforated facing sheet adhered to the porous fibrous woven material, the opposite surface of the second perforated sheet adhered to one surface of a second honeycomb core and the other surface of the second honeycomb core adhered to a third perforated facing sheet with a second layer of porous fibrous material secured to the outside or opposite surface of the third perforated sheet. A continuous communication at a predetermined flow resistance between the cells of the first and second cores and the outer surface of the second layer of porous fibrous woven material which is positioned adjacent the noise to be attenuated is maintained. The attenuation structure is specifically suitable for use in a severe environment, such as, high speed gas flow surfaces of modern aircraft. The cells of the first and second cores may be of the same or different volumes to attenuate various different noise frequencies.
In manufacturing sound attenuation metal honeycomb sandwich panels which are exposed to the above mentioned extreme environment while exposed to the sound produced by modern turbine aircraft engines, it is common practice to provide a cellular structure utilizing the Helmholtz resonating cavity principle, wherein a first thin imperforate sheet of material is bonded to one core surface of a sheet of cellular core material and a thin perforate sheet of material is bonded to the opposite core surface.
Panels of this type of construction although satisfactory for a certain degree of sound attenuation for a narrow range of sound frequencies, are found to be inefficient noise attenuation structure both for level of attenuation and for broad band noise frequencies customarily encountered in and around modern aircraft jet engines. Additionally it has been found that the perforations through the perforated sheet exposed directly to a high speed flow of air thereacross creates some turbulence to that air flow and the acoustic performance is generally effected by the grazing air flow.
Other concepts have been included interposing a sheet of fibrous material between the perforated sheet and the core surface. This has proven to be unsound acoustically when used adjacent to high speed gas flow.
Attempts to successfully manufacture this and various other adhesive bonded sandwich sound attenuation material of this general type have resulted in the adhesive used for the bonding to ooze or wick into the perforations and at least partially filling some of these perforations reducing the effective open area which increases the flow resistance between the sound source and the resonating cavities formed by the core cells. When the number and size of the perforations are increased to overcome this deficiency, the structural strength is reduced and air flow turbulence is increased. In those structures where porous fibrous material is utilized within the sandwich between the outer perforate surface and the central core, the adhesive is found to wick by capillary action into the pores and around the fibers of the porous fibrous material as well as the perforations through the perforated material further reducing the sound attenuation effectiveness of the resulting structure.